Saturday, October 6, 2012

Three papers on the Antarctic ice

Abstract: Once
thought to be devoid of life, the ice-covered parts of Antarctica are
now known to be a reservoir of metabolically active microbial cells and
organic carbon1.
The potential for methanogenic archaea to support the degradation of
organic carbon to methane beneath the ice, however, has not yet been
evaluated. Large sedimentary basins containing marine sequences up to 14kilometres thick2 and an estimated 21,000 petagrams (1Pg equals 1015g)
of organic carbon are buried beneath the Antarctic Ice Sheet. No data
exist for rates of methanogenesis in sub-Antarctic marine sediments.
Here we present experimental data from other subglacial environments
that demonstrate the potential for overridden organic matter beneath
glacial systems to produce methane. We also numerically simulate the
accumulation of methane in Antarctic sedimentary basins using an
established one-dimensional hydrate model3 and show that pressure/temperature conditions favour methane hydrate formation down to sediment depths of about 300metres in West Antarctica and 700metres
in East Antarctica. Our results demonstrate the potential for methane
hydrate accumulation in Antarctic sedimentary basins, where the total
inventory depends on rates of organic carbon degradation and conditions
at the ice-sheet bed. We calculate that the sub-Antarctic hydrate
inventory could be of the same order of magnitude as that of recent
estimates made for Arctic permafrost. Our findings suggest that the
Antarctic Ice Sheet may be a neglected but important component of the
global methane budget, with the potential to act as a positive feedback
on climate warming during ice-sheet wastage.

Dynamics of the last glacial maximum Antarctic ice-sheet and its response to ocean forcing -- Fogwill et al (2012)

Abstract: Retreat of the Last Glacial Maximum (LGM)
Antarctic ice sheet is thought to have been initiated by changes in
ocean heat and
eustatic sea level propagated from the Northern
Hemisphere (NH) as northern ice sheets melted under rising atmospheric
temperatures.
The extent to which spatial variability in ice
dynamics may have modulated the resultant pattern and timing of decay of
the
Antarctic ice sheet has so far received little
attention, however, despite the growing recognition that dynamic effects
account
for a sizeable proportion of mass-balance changes
observed in modern ice sheets. Here we use a 5-km resolution
whole-continent
numerical ice-sheet model to assess whether
differences in the mechanisms governing ice sheet flow could account for
discrepancies
between geochronological studies in different parts
of the continent. We first simulate the geometry and flow
characteristics
of an equilibrium LGM ice sheet, using
pan-Antarctic terrestrial and marine geological data for constraint,
then perturb the
system with sea level and ocean heat flux increases
to investigate ice-sheet vulnerability. Our results identify that
fast-flowing
glaciers in the eastern Weddell Sea, the Amundsen
Sea, central Ross Sea, and in the Amery Trough respond most rapidly to
ocean
forcings, in agreement with empirical data. Most
significantly, we find that although ocean warming and sea-level rise
bring
about mainly localized glacier acceleration,
concomitant drawdown of ice from neighboring areas leads to widespread
thinning
of entire glacier catchments—a discovery that has
important ramifications for the dynamic changes presently being observed
in modern ice sheets.

When that ice melted previously, global carbon dioxide levels rose dramatically over only two hundred years.

Abrupt change in atmospheric CO2 during the last ice age – Ahn et al. (2012)Abstract: “During the last glacial period
atmospheric carbon dioxide and temperature in Antarctica varied in a
similar fashion on millennial time scales, but previous work indicates
that these changes were gradual. In a detailed analysis of one event we
now find that approximately half of the CO2 increase that occurred
during the 1500-year cold period between Dansgaard-Oeschger (DO) events 8
and 9 happened rapidly, over less than two centuries. This rise in CO2
was synchronous with, or slightly later than, a rapid increase of
Antarctic temperature inferred from stable isotopes.”Citation:Ahn, J., E. Brook, A. Schmittner, and
K. J. Kreutz (2012), Abrupt change in atmospheric CO2 during the last
ice age, Geophys. Res. Lett., doi:10.1029/2012GL053018.

Comment: This is all very new science, but these three very different papers with different subjects and different methods seem together to suggest a coherent narrative; ocean warming rapidly triggers widespread decay of the Antarctic ice sheets, which uncovers significant amount of carbon. That carbon makes its way into the atmosphere, in amounts significant enough to warm the climate further.

The Arctic permafrost feedback appears (to an outsider, like me) to be gaining widespread acceptance as a significant contributor to global warming both in the near term (the next century) and in the longer term (a few centuries.) Now we are trying to nail down the scale of the feedback. Meanwhile, we are starting to get some science that suggests a similar carbon-cycle feedback could unfold in the South, scale and speed unknown.

2 comments:

Given that the difference in ice covered ocean during the summers of NH and SH is rapidly disappearing, I'd say southern Greenland Sheet is the first to completely go, as it's closest to the equator (60,5°-65,5°N vs 67.5° S for the Antarctica-based continental ice sheets). However this depends on the salinity of the Arctic and the health of AMOC and the possible breaches of warmer water through the antarctic circumpolar current (ACC), that is a deep and strong current. I've been expeting to see some great icebergs from Filchner-Ronne shelf beause of the turbulence in ACC caused by the Antarctic Peninsula, but so far this hasn't happened. Great maps of antarctica there. Not pleased to hear there is a great outlet for east antarctic sheet, but that shouldnt become a problem anyday soon (lots of ice to melt at under 70° latitude still). Thanks anyway.

In Antarctica, I'm not sure which large shelf/sheet is likely the first to go. The Transarctic mountains might direct warm air flow over Ross shelf and make it faster to melt than the Ellsworth land sheet (with the PIG outlet). Ronne-Filchner is likely somewhat protected by the antarctic gyre of Weddells, so it could be Larsen stays for longer than expected. Anyway the pace of meltdown from the quaternary glaciation has so far been faster than expected (to end the note provocatively.)